The present invention relates to an apparatus for the automatic engagement and disengagement of drive components of a motor vehicle with a driven axle, particularly the drive of at least one additional axle, and locks for auxiliary gear and differential gear.
An apparatus of this type is described in German Offenlegungsschrift No. 3,225,459. In this apparatus a change is automatically made from two-wheel drive to four-wheel drive for a given time when the difference between an "actual slip value" calculated from the rotary speeds of the wheels and a "theoretical slip value" associated with the steering angle exceeds a predetermined level. In this apparatus, two-wheel drive is adopted in tight curves, that is to say for a high "theoretical slip value", in order to avoid so-called "tight corner braking". These measures are simple steps which are not sufficient for practical driving operations, and do not appear to justify the expense of such apparatus. On tight curves particularly, four-wheel drive may be especially advantageous, but this is just where it is disengaged in the known apparatus.
Deducing the "theoretical slip value" solely from the steering angle may also lead to misinterpretations, because it is entirely possible for the vehicle to slip directly forward with wheels locked, or to drift around a curve with the wheels set straight. It is also impossible in the above apparatus to choose sufficiently low threshold values for the engagement of the four-wheel drive to ensure that unnecessary engagements are are avoided, because it is necessary to take into consideration the tolerances caused by different effective wheel diameters due to different tire pressure, different tread wear, load changes etc.
The advantage of a four-wheel drive compared to a two-wheel drive is that it improves traction and increases travelling stability. However, for both two-wheel and four-wheel drives, the driver will realize that the stability limit is being exceeded only at the physical adhesion limit. In the majority of instances, however, it is then too late for a correction.
The engagement of existing four-wheel components (drive of an additional axle, locks of auxiliary gears and/or differential gears) is also advantageous when starting from a stop, when accelerating the driven wheels, and generally when the movement condition of the vehicle threatens to exceed the stability limit.
It is therefore an objective of the present invention to provide an apparatus for the automatic engagement and disengagement of drive components of a motor vehicle which is capable of generating the switching signals and performing the engagements and disengagements in the situations mentioned with the lowest possible tolerance values and tolerance ranges.
This and other objectives are achieved by providing the apparatus for engaging and disengaging of the drive components with a sensory means for sensing the rotary speeds of the wheels of the vehicle, the rotary speeds of the input shafts of the motor vehicle, and the steering angle of the vehicle. Also provided is a control means for receiving inputs from the sensory means and comparing these inputs with stored reference values. The control means then selects a switching stage during operation of the vehicle, the switching stages being various combinations of engagements of an additional axle and locks for auxiliary and differential gear.
In order to obtain maximum traction when starting, the four-wheel drive (engagement of all the drivable axles) is always engaged below an upper limit speed value subject to hysteresis. The disengagement occurs when the upper limit speed value is exceeded, provided that the wheel acceleration of a driven axle is lower than a given limit value.
When the vehicle travelling speed falls below the lower limit speed value, the four-wheel drive is engaged again, in order to ensure full traction for a possible following acceleration phase.
The following remarks refer to vehicles with rear-axle drive and engageable front-axle drive, also with lockable central, rear-axle and front-axle differentials.
During travel, the engagement of the four-wheel components may occur when the instantaneous slip value exceeds a given limit slip value. In this context, a distinction should be made between tractive slip (when the circumferential velocity of the driven wheels is greater than the travelling speed) and thrust slip.
The instantaneous slip value is derived from the wheel rotary speeds. However, in order to avoid the above-mentioned misinterpretations, the limit slip value is derived not from the steering wheel angle, but is instead associated with the vehicle travelling speed, and additionally with the difference in the rotary speeds of the front wheels when switching back.
The engagement of the four-wheel components may also occur according to the present invention when, above a second limit speed value, a quantity characteristic of the movement condition of the vehicle does not coincide with a value or value range associated with the steering angle and the vehicle travelling speed. This is important, for example, when the vehicle enters a tightening curve without longitudinal acceleration. Here the driver may be surprised by the swinging tail of the vehicle, because it is not necessary for the limit slip value to have been exceeded. Under these conditions, the steering angle value determines the critical state reliably.
To enable the limit values and tolerance ranges to be determined as accurately and as narrowly as possible respectively, an adaptively operating matching electronic system is provided which adjusts the control unit to take into account different wheel diameters and the steering angle applicable to direct forward travel.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings, which show for purposes of illustration only, an embodiment constructed in accordance with the present invention.